1. Field of the Invention
The present invention generally relates to an image forming apparatus having a double-sided image forming function, an image forming system including the image forming apparatus, and a control method and a program to be used in the image forming apparatus.
2. Description of the Related Art
As an example of such an image forming apparatus, there has been known an image forming apparatus including a sheet feeding section accommodating normal printing papers having two sides (surfaces) on which an image can be formed and an interleaf insertion section in which interleaves (slip sheets) can be set. Herein, the interleaf set in the interleaf insertion section refers to a specific paper (such as a color sheet, an index sheet and the like) to be interleaved (inserted) between the printing papers in between, for example, different print jobs.
Japanese Patent Application Publication Nos. 1993(H05)-024735 and 1999(H11)-344847 disclose image forming apparatuses in which the interleaves set in the interleaf insertion section are supplied (fed) from the interleaf insertion section based on a print job and directly loaded to the sheet discharge section without being passed through any of the transfer section and the fixing section. As a result, the interleaves are discharged into the discharge tray with no image formed thereon.
Further, Japanese Patent Application Publication No. 2003-241915 discloses a configuration in which, as interleaf data generated by an upper-level apparatus to be used for the interleaf fed from the interleaf insertion section, there are provided one page of print layout page data, which are transmitted in addition to the print layout page data to be printed on normal printing papers.
Recently, there has been known a configuration of an image forming apparatus including plural sheet feeding trays in the sheet feeding section where the papers on which images are to be formed are set. In addition, there is a demand for printing an image on the interleaves based on the interleaf data, the interleaves being known as specific papers. To that end, there has been provided an image forming apparatus having a second sheet feeding tray in addition to a first sheet feeding tray so that the interleaves can be set in the second sheet feeding tray. By having this feature, it becomes possible to perform either single-side printing or double-side printing on the interleaves supplied (fed) from the second sheet feeding tray. On the other hand, the interleaves set in the interleaf insertion section cannot be printed but are fed (discharged) only.
In a double-side printing apparatus having the interleaf insertion section and plural sheet feeding trays, there may be a method in which the interleaves are set in the interleaf insertion section even in a normal printing operation. By doing this, a tray in the interleaf insertion section may serve as an alternative tray instead of the second sheet feeding tray. That is, in this case, the interleaves set in the tray of the interleaf insertion section can be supplied (discharged) instead of the interleaves set in the second sheet feeding tray once (after) the interleaves set in the second sheet feeding tray have been completely used up. In this double-side printing apparatus, a case is considered where an interleaf print data to be used for printing images on the interleaf set in the second sheet feeding tray are transmitted when there is no interleaf remained in the second sheet feeding tray. In this case, an interleaf set in the interleaf insertion section is fed and discharged with no image printed thereon based on the interleaf print data. As a result, no image is printed on any surface of the interleaf and the interleaf print data may be lost (without being used). However, this method has been practically used when, for example, it is desired to continue a print job to print images on the printing papers by using the interleaves set in the interleaf insertion section even when the interleaves set in the second sheet feeding tray have been completely used up. However, as described above, in a case where the print job is continued by using the interleaves set in the interleaf insertion section even when the interleaves set in the second sheet feeding tray have been completely used up, when a double-sided print job is being assigned to the interleaves set in the second sheet feeding tray, the double-sided print job is also assigned to the interleaves set in the interleaf insertion section. When the double-sided print job is assigned, two pages of page data corresponding to a data of the front and the rear surfaces are accordingly assigned to the interleaf set in the interleaf insertion section. To respond to this case, there is a disclosed method for a printing apparatus according to Japanese Patent Application Publication No. 2005-067757, in which a first interleaf is discharged based on the page data of the front surface of the double-sided print job and then a second interleaf is discharged based on the page data of the rear surface of the double-sided print job. However, when this method is applied, two interleaves are discharged based on the data of the front and the rear surfaces of the double-sided print job, which may indicate a problem that two interleaves are necessary to be discharged (used) even in a case where one interleaf is good enough.